New digital cellular communication systems, such as the Wideband Code Division Multiple Access (WCDMA) extension of the Global System for Mobile Communication (GSM) and Digital Cellular System (DCS) can utilize different operating modes for the transfer of digital information. For example, digital information can be transferred using two different duplex modes, Frequency Division Duplex (FDD) and Time Division Duplex (TDD), as are known in the art, and using different operating frequency bands. Allowing operation in the different FDD and TDD modes provides more efficient spectrum utilization. In addition, a communication can share CDMA and Time Division Multiple Access (TDMA) aspects. The GSM system operates in the 900, 1800 and the Universal Mobile Telecommunications System (UMTS) operates in the 1900/2100 MHz band, while the DCS system also operates in the 1800 MHz band.
Multi-mode communication devices are designed to transmit and receive digital communications using operating systems chosen from a plurality of multiple access techniques including TDMA, CDMA, GSM, and DCS, and will combine some of these techniques and incorporate them into one communication device. The receiver portion of a multi-mode communication device for example, is similar to those which are not multi-mode but are adapted to receive a combination of signals in accordance with any of the systems above. For example, a device operating in a FDD mode can be transmitting in an uplink (UL) on one operating system and receiving on a downlink (DL) on another operating system. In addition, the device is required to occasionally monitor various channel frequencies (FDD, TDD, GSM) of these systems to look for control channels of new base stations.
Unfortunately, in those cases where the monitoring (receiving) frequency is close to the uplink transmission frequency (i.e., frequencies in the TDD or GSM/DCS 1800/1900 MHz bands), the communication device can actually interfere with itself. For example, the transmit power of the device transmitting in DCS mode (1800 MHz) is picked up by, and interferes with, the receiver of the device, which degrades its sensitivity of WCDMA signals in the UMTS band (2100 MHz). This degradation is due to undesired co-channel noise present at the antenna as a byproduct of the amplification of the transmit signal or the transmit signal itself acting as an out of band blocker. While in a GSM call in the DCS band, an accurate evaluation of a neighbor cell in the UMTS band, having moderate to low received signal strength at the communication device, is not easily accomplished.
One solution to the problem is to simply not allow the DCS transmitter to be on at the same time as a WCDMA neighbor cell operating in the UMTS band is being evaluated by the communication device. While this solution is effective in reducing the problem somewhat, it is also difficult to accomplish since the two communication systems (i.e. transmission and reception systems) are not synchronized. Synchronization is an issue because WCDMA systems require decoding of a pilot channel to detect nearby cells, since simply measuring power is not sufficient. This can present problems since the pilot channel, primary synchronization channel (P-SCH), and secondary synchronization channel (S-SCH) information may not be available from the base station when the DCS transmitter is inactive. In addition, the GSM system does not allow for missed frames. Therefore, the receiver has to wait for another frame until the WCDMA pilot information is available between GSM transmit slots. In order to do this correctly, the device must have knowledge of the system timing of the neighbor cell before trying to decode it, and even then there are only specific times when the two systems are not colliding. By not allowing the WCDMA neighbor cell operations to occur while the DCS transmitter is on, the neighbor cell monitoring efficiency is degraded compared to the single mode case.
Another solution to prevent self-interference is to put a filter in the DCS transmit path to eliminate the noise in the WCDMA receiver band. However, due to the proximity of the band, a high-order filter with excessive loss must be used. This causes an increase in power demanded from the transmitter power amplifier. The results of these inefficiencies are higher current demanded from the battery and excessive heat generated by the transmitter circuitry. Moreover, the filter adds cost to the device and an increase in printed circuit board area is needed to place the filter. Further, the filter would not be effective in all cases and does not address the blocking issue.
In practice, typical receiver circuitry in a communication device comprises two general portions: a front end portion and a back end portion. The front end portion functions to perform initial filtering, amplification of the desired bandwidth, and conversion to an intermediate frequency for further processing by the backend portion of the receiver. The backend portion converts the signal to the baseband in preparation for digital signal processing. RF signals enter the front end portion via the antenna and are transferred from the front end to the back end. Controlling the incoming signal power to an RF receiver is essential to maintain signal levels within the operating range of the baseband circuitry and provide proper operation of the receiver. Out-of-band signal power degrades receiver performance as a result of a decreasing signal-to-noise ratio and receiver selectivity. This may occur when interfering adjacent signals are very strong compared to the desired on-channel signal, such as when a device is transmitting on the uplink while monitoring on a downlink at a nearby frequency. This results in the desired on-channel signal becoming desensitized due to out-of-band noise. Therefore, it is necessary to limit the received signal power prior to the baseband circuitry and maintain signal levels within the back end circuit's operating range. Filter portions of the baseband circuitry can reduce the adjacent interference noise signals by allowing only the desired on-channel frequency to pass through. However, the incoming aggregate power level prior to the baseband circuitry comprises the desired monitored signal as well as the interfering uplink energy, which limits the usefulness of filtering.
Therefore, there is a need to alleviate the problems of sensitivity degradation during receiver monitoring in a multimode communication device. It would also be of benefit to increase the efficiency of neighboring cell monitoring without requiring timing information of both systems simultaneously. It would also be advantageous to provide these improvements without any significant additional hardware or cost in the communication device.